For nearly a century the Weston-type load brake has been widely employed as a load control device for hoisting machines, to prevent descent of the load when the drive motor for the machine is delivering no torque to the cable drum and to control descent of the load when the motor is operating in the lowering direction. Through the years there have been many modifications and proposed modifications in the Weston load brake, as exemplified by U.S. Pat. No. 4,009,770, issued in 1977; but in essentials it has not changed, nor have all of its inherent disadvantages been overcome.
In general, a load brake of the Weston-type is characterized by a ratchet wheel that is permitted to rotate during raising of a load but is at all other times held against rotation by a pawl. In a simple form of Weston load brake, the load is coupled to a helically splined or threaded shaft that extends coaxially through the ratchet wheel and is freely rotatable relative to it. At one axial side of the ratchet wheel there is a flange-like friction element that is secured to the threaded shaft to rotate with it. At the other side of the ratchet wheel there is an axially movable friction element which has an internal thread engaged with the helical thread on the shaft and which is constrained to rotate with the drive motor.
When the drive motor is at rest so that the axially movable friction element is not being rotated, load torque imposed upon the threaded shaft tends to rotate that shaft in a direction such that the axially movable friction element is displaced axially along the shaft into engagement with the ratchet wheel, which is in turn forced axially into engagement with the flange-like friction element on the threaded shaft. Since the ratchet wheel is confined against load lowering rotation by the pawl, the clampwise engagement of the ratchet wheel by the friction elements prevents the threaded shaft from rotating in the load lowering direction in response to the torque that the load imposes upon it. If the motor now applies torque to the axially movable friction element in the load raising direction, any tendency towards relative rotation between that friction element and the load-biased threaded shaft will effect tighter clamping of the ratchet wheel by the friction elements; but since the ratchet wheel can turn freely in the load raising direction, friction due to such clamping merely results in more secure coupling of the axially movable friction element to the flange-like friction element so that the load is raised without slippage. When the hoisting motor is operated in the load lowering direction, the axially movable braking element is driven in the direction such that its rotation relative to the threaded shaft tends to carry it axially away from the clamping relationship. The ratchet wheel continues to be confined against rotation by the pawl, but to the extent that clamping force of the friction elements against the ratchet wheel is relieved, the flange-like element--and hence the threaded shaft--can rotate under load bias. However, the torque that the load imposes upon the threaded shaft still tends to drive it in the direction that actuates the axially movable friction element towards clamping engagement with the ratchet wheel, and therefore the load descends at a controlled rate that corresponds to the rotational speed of the drive motor, under a condition of equilibrium in which load torque plus motor torque are opposed by an equivalent drag force due to friction between the friction elements and the ratchet wheel.
The need for a pawl and ratchet arrangement in a conventional Weston load brake can be seen as a disadvantage because it requires the relatively expensive cutting of a ratchet gear. Heretofore, however, the necessity for such a mechanism was regarded as practically unavoidable, since the known alternatives (which included one-way clutches and various cam arrangements) tended to be more costly and/or less sturdy and reliable for use with very heavy loads.
A more fundamental objection to the Weston load brake was that it tended to impose limits upon the performance of hoisting apparatus driven by an electric motor having a given starting torque. As can be seen from the above description of the Weston device, the initial application of lifting torque to the axially movable friction element tended to drive it axially into firm engagement with the ratchet wheel. If the drive motor was capable of imparting a substantially high upward acceleration to a heavy load, the clamping engagement of the friction elements with the ratchet wheel could become so tight that the axially movable friction element acted upon the threaded shaft like a jam nut. When this happended, the motor did not have enough starting torque to break the jam and start the shaft rotating in the lowering direction.
Because of this tendency for the Weston load brake to lock up against load lowering, the motor that was selected for a given hoisting machine had to be one that had a higher starting torque than was actually needed for satisfactory load lifting acceleration, and the machine had to be restricted against use with loads that were actually within the load torque capabilities of its motor.
The helical power thread on the shaft of a Weston-type load brake was expensive to produce and was disproportionately more expensive with larger shaft sizes. The shaft was therefore kept as small as possible. To that end, the load brake was usually connected directly to the drive motor, and the conventional reduction gear train was connected between the load brake and the cable drum. The load brake was thus subjected to relatively small forces, but it operated at relatively high speed and was often so associated with the gear train that its rotating parts had to be lubricated. Since a load brake is dependent upon friction for its operation, severe design and maintenance problems were posed by the need for maintaining frictional relationships while at the same time providing for adequate lubrication. There was also some potential hazard in the interposition of the reduction gear train between the load brake and the cable drum because a failure in the gear train could result in the load being free to descend unrestrainedly.
Another potentially hazardous feature of the Weston-type load brake was that the helical power threads on its shaft were in certain cases subject to fatigue failure; and if they failed, there was nothing to restrain the load against descent. It was of course customary to provide an electromagnetically actuated brake that held the drive motor shaft against rotation at any time that the motor was not energized, but failure of the helical threads effectively uncoupled the cable drum from the motor shaft so that the electromagnetic brake had no control over cable drum rotation.